How does the presence of a stigma in the Euglena help it to capture light energy?

In Euglena the kight sensor is found in the parabasal body, a swelling at the flagellar basis. The stigma functions as a sun-shade, throwing a shadow on the parabasal body. Since the cells do permanently rotate around their own axis during movements, is the parabasal body periodically exposed to light or shaded from it. The cell is thus able to detect the direction of the light and to react to it by moving towards it = positive phototaxis.